Optical disks functioning as an optical recording element from which information is played back with a light beam have been already known as a high density and high storage capacity recording medium. Of these optical disks, optical disks used only for the playback of information such as optical ROM disks are particularly widely known. In an optical ROM disk, application software for a computer and various data are already stored in the form of pit alignments on the disk. A data retrieval system for use with such an optical ROM disk is designed such that the presence/absence of a pit is detected by scanning the pit alignments with a laser beam which is used as a light beam; the presence/absence of a pit is electrically represented by a binary signal; and stored information is played back in accordance with the binary signal.
More concretely, as shown in FIG. 9(a), when a laser beam spot scans the alignment of pits 42 formed on a track 43 on an optical disk 41, the intensity of reflected light decreases as the laser beam enters the pit 42 due to light diffraction. The electric output of a read-out signal is changed in accordance with the presence/absence of the pits 42, namely, it decreases when the laser beam scans the pit 42, and increases when the laser beam scans the flat area where no pit 42 is formed, as shown in FIG. 9(b). The output of the signal is converted to a binary code with a predetermined threshold value to obtain a binary signal as shown in FIG. 9(c).
In the conventional optical disk 41, information recorded by the pit 42 is represented by 0 or 1 and therefore the information volume thereof is not more than 1 bit. In order to enable optical disks to perform higher-density recording and have higher storage capacity, the length of the smallest pits has to be shortened to reduce the spacing between each pit, or the track pitch has to be reduced. This, however, requires an improvement in the frequency characteristic of the optical head to a considerable extent and causes more cross-talk in reading out signals. In other words, the use of an optical disk in which the pit spacing and track pitch are reduced in order to achieve higher-density and large-volume recording, has the disadvantage of overloading the optical disk data retrieval system used for driving such a disk.